1. Field of the Invention
The present invention relates to a semiconductor integrated circuit designed for testability and to a test method for the same and, more particularly, to a semiconductor integrated circuit incorporating a test configuration having a scan chain constructed by connecting a plurality of scan flip-flops in a chain, and a test method for such a semiconductor integrated circuit.
2. Prior Art
With advances in deep submicron processes and multilayered wiring structures, semiconductor chips such as VLSI devices (very large-scale semiconductor integrated circuit devices) have been greatly improved both in performance and in functionality. On the other hand, the difficulty in testing such VLSI devices has become a major problem. Therefore, in VLSI devices with increased integration and increased operating speed, there has developed a need for detecting new kinds of faults (crosstalk, etc.) at lower cost.
FIG. 1 shows an LSI test method. A stuck-at fault shown in FIG. 1A is a typical VLSI fault model. This fault is a fault that causes the input or output of a device in a circuit to permanently take a logic value 0 or 1. In the example shown here, the output signal line of a NAND gate is in a stuck-at-0 fault condition. When inputs are applied that should cause the NAND gate to output a 1 on the signal line, the internal state of the circuit becomes different from normal. In the illustrated example, the influence of the fault propagates outside the circuit. That is, when a vector {1, 0, 1} is applied, if the output is a 1, this means that the signal line is in a stuck-at-0 fault condition; on the other hand, if the output is 0, this means that the signal line is not in a stuck-at-0 fault condition. Accordingly, the vector {1, 0, 1} is a test vector for detecting a stuck-at-0 fault on the signal line.
With increasing semiconductor integration and process miniaturization, there has been a growing need to detect faults that cannot be detected using single stuck-at fault models only. One technique that is expected to be effective in detecting the so-called “unmodeled faults” is the n-detection test shown in FIG. 1B. The n-detection test is a method that detects a single stuck-at fault by performing the detection a plurality of times using different test vectors. In the illustrated example, three vectors are the test vectors all of which are applied to detect the same fault. With the n-detection test, it becomes possible to detect a fault that could not be detected by a single detection test. The method is also expected to be effective in detecting the so-called “unmodeled faults” such as crosstalk and delay faults.
However, the n-detection test requires the use of many test vectors. In the example of FIG. 1B, three test vectors are applied in order to detect a stuck-at-0 fault. In this way, to perform the n-detection test, the amount of test data increases as the number of detections increases, and a technique for reducing the amount of the test data is essential in order to perform the test efficiently.
Further, the amount of test data and the test time required to test VLSI devices have been increasing year by year. There are cases where the test data generated by an ATPG (Automatic Test Pattern Generator) exceeds the memory capacity of a semiconductor tester. Furthermore, as the operating speeds of VLSI devices increase, there occur cases where tests cannot be done using existing testers. There is therefore a need to test high-speed VLSI devices using low-speed testers.
As one method for performing the n-detection test at low cost, there is proposed a method that uses a partially rotational scan (PRS) circuit (refer to document 1 listed below). The partially rotational scan circuit is a technique employed for design for testability, and makes at-speed testing possible by using a low-speed tester when performing the n-detection test; with this circuit, many shift vectors other than the ATPG vectors can be generated by partially rotating the scan chain. Using these shift vectors, the amount of test data can be reduced. Since the test data need not be scanned in during the rotation operation, at-speed testing can be performed using a low-speed tester.
To fabricate a VLSI device incorporating a test configuration that uses the partially rotational scan circuit, a test response compactor for extracting responses from the circuit-under-test (CUT) must be added to the circuit-under-test in addition to the partially rotational scan circuit that generates test patterns. A multiple-input signature register (MISR) is often used as the test response compactor.
FIG. 2 shows an LSI test configuration that uses the partially rotational scan (hereinafter abbreviated PRS) circuit. In the figure, reference numeral 1 is a tester which generates test vectors, 2 is the PRS circuit, 3 is the circuit-under-test (hereinafter abbreviated CUT), and 4 is a MISR as the test response compactor. The PRS circuit 2, the CUT 3, and the MISR 4 are integrated as a single LSI device. The test vectors generated by the tester 1 are input to the PRS circuit 2 which then generates shift vectors and rotation vectors, thereby making it possible to reduce the amount of the test data. Accordingly, a low-speed tester can be used as the tester 1. Test results are compacted by the MISR 4, and output from the integrated circuit device.
(Document 1) “Application of Partially Rotational Scan Technique with Tester IP for Processor Circuits,” IEICE Trans. Inf. & Syst., Vol. E87-D, No. 3, pp. 586-591 (Mar. 2004), Kenichi Ichino, Ko-ichi Watanabe, Masayuki Arai, Satoshi Fukumoto, and Kazuhiko Iwasaki.
In the integrated semiconductor circuit incorporating the test configuration that uses the PRS circuit shown in FIG. 2, as the PRS circuit 2 generates the rotation vectors by performing the rotation operation with the shift vectors input thereto, the results of the testing of the CUT 3 cannot be held therein, unlike the conventional scan circuit. Accordingly, the MISR circuit 4 is added to hold the test results. However, the addition of the MISR circuit 4 results in a corresponding increase in the VLSI circuit area. Furthermore, selectors, etc. added in the PRS circuit 2 also increase the circuit area, and as a result, this test configuration has the shortcoming that area overhead significantly increases compared with the conventional scan-type test configuration. Accordingly, in the test configuration that uses the PRS circuit, a major technical challenge is how the area overhead can be reduced in the fabrication of the semiconductor device.